120 THE CONTRACTILE TISSUES. 



as normal saline solution, isolated cells or small groups of cells may be seen 

 rowing themselves about, as it were, by the action of their cilia. 



All the cilia of a cell move, as we have just said, in the same direction, 

 but not quite at the same time. If we call the side of the cell toward which 

 the cilia bend the front of the cell and the opposite side the back, the cilia 

 at the back move a trifle before those at the front, so that the move- 

 ment runs over the cell in the direction of the movement itself. Sim- 

 ilarly, taking any one cell, the cilia of the cells behind it move slightly 

 before and the cilia of the cells in front of it slightly after its own cilia 

 move. Hence, in this way, along a whole stretch of epithelium, the 

 movement or bending of the cilia sweeps over the surface in ripples or 

 waves, very much as, when the wind blows, similar waves of bending sweep 

 over a field of corn or tall grass. By this arrangement the efficacy of the 

 movement is secured, and a steady stream of fluid carrying particles is 

 driven over the surface in a uniform continued direction ; if the cilia of 

 separate cells, and still more if the separate cilia of each cell, moved 

 independently of the others, all that would be produced would be a series 

 of minute " wabbles," of as little use for driving the fluid definitely on- 

 ward as the efforts of a boat's crew all rowing out of time are for propelling 

 the boat. 



Swift bending and slower straightening is the form of ciliary movement 

 generally met with in the ciliated epithelium of mammals and, indeed, of 

 vertebrates ; but among the invertebrates we find other kinds of movement, 

 such as a to-and-fro movement, equally rapid in both directions, a cork- 

 screw movement, a simple undulatory movement, and many others. In 

 each case the kind of movement seems adapted to secure a special end. 

 Thus even in the mammal while the one-sided blow of the cilia of the epi- 

 thelial cells secures a flow of fluid over the epithelium, the tail of the sper- 

 matozoon, which is practically a single cilium, by moving to and fro in an 

 undulatory fashion drives the head of the spermatozoon onward in a straight 

 line, like a boat driven by a single oar worked at the stern. 



Why and exactly how the cilium of the epithelial cells bends swiftly and 

 straightens slowly, always acting in the same direction, is a problem dif- 

 ficult at present to answer fully. Some have thought that the body of 

 the cell is contractile, or contains contractile mechanisms pulling upon the 

 cilia, which are thus simple passive puppets in the hands of the cells. But 

 there is no satisfactory evidence for such a view. On the whole the evi- 

 dence is in favor of the view that the action is carried out by the cilium 

 itself, that the bending is a contraction of the cilium, and that the straight- 

 ening corresponds to the relaxation of a muscular fibre. But even then 

 the exact manner in which the contraction bends and the relaxation 

 straightens the filament is not fully explained. We have no positive evi- 

 dence that a longitudinal half, the inside we might say, of the filament is 

 contractile, and the other half, the outside, elastic, a supposition which has 

 been made to explain the bending and straightening. In fact, no adequate 

 explanation of the matter has as yet been given, and it is really only on 

 general grounds we conclude that the action is an effect of contractility. 



In the vertebrate animal cilia are, as far as we know, wholly independent 

 of the nervous system, and their movement is probably ceaseless. In such 

 animals, however, as infusoria, hydrozoa, etc., the movements in a ciliary 

 tract may often be seen to stop and to go on again, to be now fast, now slow, 

 according to the needs of the economy, and, as it almost seems, according to 

 the will of the creature ; indeed, in some of these animals the ciliary move- 

 ments are clearly under the influence of the nervous system. 



Observations with galvanic currents, constant and interrupted, have not 



